Academic literature on the topic 'Kendall et Roberts'

The Weierstrass series was considered in Ciavarella et al. (Ciavarella et al. 2000 Proc. R. Soc. Lond. A 456 , 387–405. ( doi:10.1098/rspa.2000.0522 )) to describe a linear contact problem between a rigid fractally rough surface and an elastic half-plane. In such cases, no applied mean pressure is sufficiently large to ensure full contact, and specifically there are not even any contact areas of finite dimension. Later, Gao & Bower (Gao & Bower 2006 Proc. R. Soc. A 462 , 319–348. ( doi:10.1098/rspa.2005.1563 )) introduced plasticity in the Weierstrass model, but concluded that the fractal limit continued to lead to what they considered unphysical predictions of the true contact size and number of contact spots, similar to the elastic case. In this paper, we deal with the contact problem between rough surfaces in the presence of adhesion with the assumption of a Johnson, Kendall and Roberts (JKR) regime. We find that, for fractal dimension D >1.5, the presence of adhesion does not qualitatively modify the contact behaviour. However, for fractal dimension D <1.5, a regularization of the contact area can be observed at a large magnification where the contact area consists of segments of finite size. Moreover, full contact can occur at all scales for D <1.5 provided the mean contact pressure is larger than a certain value. We discuss, however, the implication of our assumption of a JKR regime.

Summary Waterflooding recovers little oil from fractured carbonate reservoirs, if they are oil-wet or mixed-wet. Surfactant-aided gravity drainage has the potential to achieve significant oil recovery by wettability alteration and interfacial tension (IFT) reduction. The goal of this work is to investigate the mechanisms of wettability alteration by crude oil components and surfactants. Contact angles are measured on mineral plates treated with crude oils, crude oil components, and surfactants. Mineral surfaces are also studied by atomic force microscopy (AFM). Surfactant solution imbibition into parallel plates filled with a crude oil is investigated. Wettability of the plates is studied before and after imbibition. Results show that wettability is controlled by the adsorption of asphaltenes. Anionic surfactants can remove these adsorbed components from the mineral surface and induce preferential water wettability. Anionic surfactants studied can imbibe water into initially oil-wet parallel-plate assemblies faster than the cationic surfactant studied. Introduction Waterflooding is an effective method to improve oil recovery from reservoirs. For fractured reservoirs, waterflooding is effective only when water imbibes into the matrix spontaneously. If the matrix is oil-wet, the injected water displaces the oil only from the fractures. Water does not imbibe into the oil-wet matrix because of negative capillary pressure, resulting in very low oil recovery. Thus there is a need of tertiary or enhanced oil recovery techniques like surfactant flooding (Bragg et al. 1982; Kalpakci et al. 1990; Krumrine et al. 1982a; Krumrine et al. 1982b; Falls et al. 1992) to maximize production from such reservoirs. These techniques were developed in 1960s through 1980s for sandstone reservoirs, but were not widely applied because of low oil prices. Austad et al. (Austad and Milter 1997; Standnes and Austad 2000a; Standnes and Austad 2000b; Standnes and Austad 2003c) have recently demonstrated that surfactant flooding in chalk cores can change the wettability from oil-wet to water-wet conditions, thus leading to higher oil recovery (~70 % as compared to 5% when using pure brine). In 2003 (Standnes and Austad 2003a; Standnes and Austad 2003b; Strand et al. 2003), they identified cheap commercial cationic surfactants, C10NH2 and bioderivatives from the coconut palm termed Arquad and Dodigen (priced at US$ 3 per kg). These surfactants could recover 50 to 90% of oil in laboratory experiments. However, the cost involved is still high because of the required high concentration (~1 wt%) and thus there is a need to evaluate other surfactants. The advantage of using cationic surfactants for carbonates is that they have the same charge as the carbonate surfaces and thus have low adsorption. Nonionic surfactants and anionic surfactants have been tested by Chen et al. (2001) in both laboratory experiments and field pilots. Computed tomography scans revealed that surfactant imbibition was caused by countercurrent flow in the beginning and gravity-driven flow during the later stages. The basic idea behind these techniques is to alter wettability (from oil-wet to water-wet) and lower interfacial tension. Hirasaki and Zhang (2004) have studied different ethoxy and propoxy sulfates to achieve very low interfacial tension and alter wettability from oil-wet to intermediate-wet in laboratory experiments. The presence of Na2CO3 reduces the adsorption of anionic surfactant by lowering the zeta potential of calcite surfaces, and thus dilute anionic surfactant/alkali solution flooding seems to be very promising in recovering oil from oil-wet fractured carbonate reservoirs. It is very important to understand the mechanism of wettability alteration to design effective surfactant treatments and identify the components of oil responsible for making a surface oil-wet. It is postulated that oil is often produced in source rocks and then migrates into originally water-wet reservoirs. Some of the ionic/polar components of crude oil, mostly asphaltenes and resins, collect at the water/oil interface (Freer et al. 2003) and adsorb onto the mineral surface, thus rendering the surface oil-wet. In this work, we try to understand the nature of the adsorbed components by AFM. Recently, AFM has been used extensively to get the force-distance measurements between a tip and a surface. These force measurements can be used to calculate the surface energies using the Johnson-Kendall-Roberts (JKR), the Derjaguin-Muller-Toporov (DMT), and like theories (van der Vegte and Hadziioannou 1997; Schneider et al. 2003). AFM is also used extensively for imaging surfaces. It can be used in the contact mode for hard surfaces and in the tapping mode for soft surfaces. It can be used to image dry surfaces or wet surfaces; tapping mode in water is a relatively new technique. AFM images have been used to confirm the deposition of oil components on mineral surfaces (Buckley and Lord 2003; Toulhoat et al. 1994). In this work, crude-oil-treated mica surface is probed using atomic force microscopy before and after surfactant treatment to study the effects of surfactant. AFM measurements are correlated with contact-angle measurements. We also study surfactant solution imbibition into an initially oil-wet parallel plate assembly to relate wettability to oil recovery. Our experimental methodology is described in the next section, the results are discussed in the following section, and the conclusions are summarized in the last section.

The Johnson–Kendal–Roberts (JKR) adhesion theory is frequently applied to extract the surface energy of the contacting thin coating systems in micro or nanoprobe instruments such as Surface Force Apparatus (SFA) and Atomic Force Microscope (AFM). For thin-layer systems, the JKR theory may give rise to erroneous predictions as it is based on the elastic contact between a sphere and a half-space. Adhesion between the thin-layer surfaces has been analyzed by Sridhar et al. using a numerical SJF (Sridhar–Johnson–Fleck) model. In this paper, the adhesion between a spherical tip of an AFM and an incompressible thin elastic film is investigated. When the substrate is rigid, the normalised pull-off force may differ from the JKR value of -0.5 by as much as 90%. Computations of the contact size and pull-off force are presented for a range of values of adhesion energy. Finally, an empirical equation for the adhesive load was developed by curve fitting the compliance of the layer system as a function of contact radius.

In autumn 1996, under the Presidency of Dr Robert Kendell, the College decided to mount a campaign to tackle the stigmatisation of people with mental illnesses. In 1997, a working party proposed goals, content, process and a 5-year governance. Other campaigns, both here and abroad, have either generically addressed ‘mental health problems' (e.g. Mind's ‘Respect’ Campaign) or targeted a specific mental illness, e.g. the World Psychiatric Association's anti-stigma campaign in respect of people with schizophrenia. Our working party decided that it might be timely to recognise the differences in public attitudes to the variety of mental illnesses. The campaign thus addressed six categories of mental illness: anxiety disorders, depressive disorders, schizophrenia, the dementias, eating disorders, and drug and alcohol misuse/addiction. Target populations were identified as doctors, children and adolescents, the workplace, the media and the general public. The working party had also secured funding and arranged for a survey, in July 1998, by the Office for National Statistics of opinions of the British public concerning people with these mental illnesses (Crisp et al, 2000). The Campaign started on 7 October 1998. Since that time, and drawing upon our survey findings within its initial literature review (Kelly, 1999) the Department of Health mounted its own anti-stigma campaign, ‘Mind Out for Mental Health’, which addressed a similar range of mental illnesses.

AbstractIn this study, we investigate the effects of thermal fluctuations on the generalized Johnson–Kendall–Roberts (JKR) model. We show that the distribution of pull-off forces in this model is similar to that of the Bradley model, and is also consistent with the experiment result observed in Wierez-Kien et al. (Nanotechnology 29(15):155704 2018). Increasing temperature leads to a broadening of the distribution, while leads to a reduction of the pull-off force. Additionally, the pull-off force, which is separated into an athermal term and a thermal-induced reduction term, is measured by using spring velocity ranging over 5 orders of magnitude. We show that for compliant spring, the pull-off force is significantly enhanced with increasing velocity, which is mainly attributed to the contribution of the thermal-induced reduction term, while the athermal term is barely sensitive to changes in velocity.

The plane strain adhesive contact between a periodic wavy surface and a flat surface has been revisited based on the classical Maugis–Dugdale model. Closed-form analytical solutions derived by Hui et al. [1], which were limited to the case that the interaction zone cannot saturate at a period, have been extended to two additional cases with adhesion force acting throughout the whole period. Based on these results, a complete transition between the Westergaard and the Johnson, Kendall, and Roberts (JKR)-type contact models is captured through a dimensionless transition parameter, which is consistent with that for a single cylindrical contact. Depending on two dimensionless parameters, different transition processes between partial and full contact during loading/unloading stages are characterized by one or more jump instabilities. Rougher surfaces are found to enhance adhesion both by increasing the magnitude of the pull-off force and by inducing more energy loss due to adhesion hysteresis.

Online life is embedded within the complexes, organisations, power ratios, and conceptualisations of offline life. The ambiguities of the interaction between online and offline - the testing and questioning, asking and affirming of their match - make a vital part of their current phenomenology. One factor which seems to exert resistance - some form of implacability - however unexpectedly, is that of gender. Other categories, such as politics and nationality, seem to have similar, but more direct, effects. Online folklore is quite specific, and the folklore can be academic as analysts are affected by the same forces and experiences as other people. Firstly, it is claimed that gender is unimportant online - that you are free to be yourself, or whomsoever you choose. Mark Poster, who is not alone, claims: "one may experience directly the opposite gender by assuming it and enacting it in conversations" (1997: 223), which seems to confuse a temporary imagining with a long term imposed. Secondly, however, there are the repeated tales that online life is full of cross-gender impersonation and that, as a result, it is full of potential betrayal and disillusionment. Kolko writes: The stories of online cross-dressing that abound… often culminate in narratives of betrayal. In this accumulated body of scholarship, participants talk of how their notions of the world and their selves and others has been destabilized, rocked beyond recognition, until they are left feeling adrift, at sea, that they 'cannot trust anyone,' that 'everything online can be a lie,' that 'no one tells who they really are.' (1997: np). These stories seem a little contradictory. If gender is unimportant online, then how come it is such a source of anxiety? Often the anxiety seems concealed under a discourse of futurity, in which attention is given to what life online might become - with contradictions deleted - rather than giving attention to what actually happens or has happened. This move maintains the theory that the Internet is free of the effects of offline life, perhaps in compensation for increasing restrictions and insecurities in offline life. It also keeps the Internet in the realm of science fiction, where it is without history, and is cutting edge by default. People generally seem sure they can detect the gender of others (Suler 1996). A person might use a lot of emoticons and be female (Witmer & Katzman 1998: 6, 9), they might flame and be male (Baym 1995: 158, Herring 1994), they might not know American pantyhose sizes and be male (irrespective of if they come from a country which uses different sizings - most of the rest of the world - or are a woman who does not wear pantyhose). People read books by academics describing how the sexes use language differently in order to detect these differences - while, at the same time, others read them in order to fake better (Wright 2000). If people select gender neutral pronouns in a MOO, then others will not assume they do not have a gender, they will generally attempt to work it out (Kendall 1996: 217), and some research suggests that if a person refuses to reveal their offline gender then they will be dropped from interactions (O'Brien 1999: 90). Most of these ways of determining 'real gender', use offline gender clichés to make that detection, or to manufacture that production. Thus McRae, after arguing in favour of absolute gender freedom, remarks that if someone plays a woman and wants to "attract partners as 'female' [they] must craft a description within the realm of what is considered attractive" (1996: 250). They are likely to exaggerate conventions of gender (as with plastic surgery). As Kendall writes: "choosing one gender or another does nothing to change the expectations attached to particular gender identifications" (1996: 217). In fact, as people online can ignore the gender of those contradicting their expectations of gender, then those expectations may grow stronger. Categories of gender might strengthen rather than weaken. A performance which follows rules and conventions in order to convey a message, which to some extent any successful performance must, may not challenge the accepted conceptual order at all. As not all the rules and conventions of performance may be conscious or understood by performers, their performance may unintentionally harden those conventions. This is especially so as in Western online social practice, whatever the complexity of our theory, gender seems to be treated as an 'essential' and equated with an offline body. Further, gender impersonation appears to happen in specific circumstances, namely on IRC, or MOOs, or in games. It rarely if ever happens on Mailing Lists, where anonymity is rarer (names are often given in addresses, and email often signed with a gendered name), and people try to manifest real authentic identities, rather than play with possible identities. An experimental mailing list I was involved in, in which members, previously known to each other from another list, were anonymous and their gender not specified, was short lived. It did not become a site of play with, or 'beyond', gender. As well, people seem only disturbed when males are thought to be impersonating females - which indicates a degree of gender specificity itself. I have never seen a list of ways to detect whether a male avatar was female, while the opposite is common. This is not just anxiety from heterosexual males seeking sex partners but, in a famous case, occurred when women found that a supposedly female confidant was male (Van Gelder 1996, Stone 1995: 69-81). So why does this happen? Answers to such questions must be provisional, but it is probably connected with the role of women, in offline life, in maintaining and marking intimacy and support. We might over-quickly say that, offline (outside of male homosexual groups in which gender may function differently), an intimate relationship usually includes one woman. Emotional or support bonding is female. Male bonding has become almost suspect, particularly if it involves intimacy. We also live in a society with a common discourse about the decline of support through kinship, a rise in single person households, increased insecurity in work, and decline in state support for people in times of stress, at the very time that kinship no longer is able to give support (Castells 1997: 97). The Western generative atom of kinship, intimacy and support, approaches being the heterosexual pair bond. Wuthnow claimed in 1994 that, faced with these kinds of insecurities, 40% of American adults became members of small groups meeting regularly to provide support for members (1994: 45-50). There is no reason to assume the percentage has declined. Internet groups can be seen to function similarly; they are means by which people make contacts, provide help, discuss problems or interests, and sometimes get work - fibreculture being a case in point. There is often, in such groups, a fairly active 'off-group' life of correspondence and contact between particular members. Structurally, life which is off a mailing list, or in a private MOO room, approaches the structure of the dyadic, private and intimate pair bond, which should contain at least one woman. Finding out that a person you have been intimate with was not female, almost automatically changes the relationship from the realm of intimate and private into a public betrayal. Our private role and its vulnerabilities has broken into the public male domain and symbolically been exposed rather than shared. Gender functions as a way in which people interpret the actions of others. As Ten Have argues (2000), categorisation is one of the ways in which people decide what kinds of messages others are emitting, whether they have much in common and whether they can be trusted. The importance of particular categories can frequently be ascertained by how often they are requested - and gender is one of the most requested when unknown and when people are seeking support or intimacy. Because authenticity, or trying to find the truth of others, is important in Western thought about relationships, this truth becomes anchored in the 'real' body and the customs surrounding it. 'Truth' is confirmed the more private the information, and the more it is received offline. As a result, and because of models of intimacy, 'real gender' then becomes important and is central to current online life. It may also point to other ways in which the tension between the online and offline manifest, or in which an apparently abstract flow meets resistance. Works Cited Baym, Nancy (1995) "The Emergence of Community in Computer-Mediated Communication", in Steven G. Jones ed. Cybersociety: Computer-Mediated Communication and Community, Sage, Thousand Oaks. Castells, Manual (1997) The Information Age: Economy, Society and Culture Volume 2: the Power of Identity, Blackwell, 1997. Cherny, Lynn & Weise, Elizabeth R. ed. (1996) Wired Women: Gender and New Realities in Cyberspace, Seal Press, Seattle. Herring, Susan (1994) "Gender Differences in Computer-Mediated Communication: Bringing Familiar Baggage to the New Frontier", Keynote talk at panel entitled Making the Net*Work*: Is there a Z39.50 in gender communication?, American Library Association annual convention, Miami, June 27, 1994. <http://cpsr.org/cpsr/gender/herring.txt> Kendall, Lori (1996) "MUDder? I Hardly Know 'Er! Adventures of a feminist MUDder" in Cherny & Weise, 1996. Kolko, Beth (1997) "Discursive Citizenship: The Body Politic in Cyberspace", paper presented at the Creative Collaboration in Virtual Communities Conference, Sydney, Australia, February 1997. <http://www.arch.su.edu.au/kcdc/conferences/VC97/papers/kolko.php> McRae, Shannon (1996) "Coming Apart at the Seams: Sex, Text and the Virtual Body" in Cherny & Weise. O'Brien, Jodi (1999) "Gender (Re)production in Online Interaction", in M.A. Smith, & P. Kollock, eds. Communities in Cyberspace, Routledge, London. Poster, Mark (1997) "Cyberdemocracy", in David Holmes ed. Virtual Politics: Identity & Community in Cyberspace, Sage Publications, London. Stone, A. R. (1995) The War of Desire and Technology at the Close of the Mechanical Age, MIT Press. Suler, John (1996) "Do Boys Just Wanna Have Fun? Male Gender-Switching in Cyberspace (and how to detect it)". <http://www.rider.edu/users/suler/psycyber/genderswap.php> ten Have, Paul (2000) " 'hi, a/s/l please?': identification/categorisation in computer mediated communication" A paper/mosaic presented at the 'Sociaal-Wetenschappelijke Studiedagen 2000', Session ICT & Huiselijk Leven. Van Gelder, Lindsay (1996) "The Strange Case of the Electronic Lover", in Rob Kling ed Computerization and Controversy: Value Conflicts and Social Choices, 2nd Edition, Academic Press, San Diego. Witmar, D.F. & Katzman, S.L. “Smile When You Say That: Graphic Accents as Gender Markers” in Fay Sudweeks et al eds. Network & Netplay: Virtual Groups on the Internet, AAAI Press, MenloPark. Wright, Kathryn (2000) "Gender Bending in Games" <http://www.womengamers.com/articles/gender.php> Wuthnow, Robert (1994) Sharing the Journey: Support Groups and America's New Quest for Community, Free Press, NY. Links http://cpsr.org/cpsr/gender/herring.txt http://www.arch.su.edu.au/kcdc/conferences/VC97/papers/kolko.html http://www.rider.edu/users/suler/psycyber/genderswap.html http://www.womengamers.com/articles/gender.html%3e Citation reference for this article Substitute your date of access for Dn Month Year etc... MLA Style Marshall, Jonathan. "Resistances of Gender" M/C: A Journal of Media and Culture< http://www.media-culture.org.au/0308/06-resistances.php>. APA Style Marshall, J. (2003, Aug 26). Resistances of Gender. M/C: A Journal of Media and Culture, 6,< http://www.media-culture.org.au/0308/06-resistances.php>

Flow evolving in turbomachinery applications is turbulent and laden with particles, such as dust, ash, dirt, etc. This would affect the behaviour of the turbine components given that solid particles can impact and then bounce off, deposit or erode their surfaces. Erosion and deposition phenomena may seriously affect the components performance, because they alter the blade profile and hence the flow field. It is thus clear that the prediction of these phenomena would be of great help form both design optimization and maintenance of turbomachinery. Besides experiments, in the last decade CFD became one of the main tool for studying flow evolution within turbine components, phenomena that involve them, and prediction of problems. In particle-laden flows, CFD is used to simulate the flow field, but also solid particle transport and dispersion, impact mechanics, adhesion or rebound, and erosion. Several approaches can be used depending on the kind of application studied and information expected. Particle transport can be simulated adopting a single or cluster of particle tracking approach (Crowe et al, 2006). Since to have a statistically independent results a large number of simulated particles is needed, the for-mer approach can be used when the domain size is not too large; moreover the instantaneous flow field is needed, otherwise turbulent dispersion of particles has to be accounted for. The cluster of particles approach (i.e., Particle Cloud Tracking model, Baxter 1989) overcomes some of these problems, since it usually uses a model for particle dispersion, computing very few trajectories to simulate a large number of particles. Particle impact/rebound and deposit/erosion are modelled using one of the available choice. For instance, impact mechanics can be modelled according to the Johnson-Kendall-Roberts theory (1971) if the particle temperature is not large enough to modify the physical properties of the particles, or the Thornton and Ning variation (1997). When the effect of temperature becomes relevant, a temperature based sticking model is used, such as that of Walsh et al. (1990). Erosion can be studied according to the model of Tabackof (1979). Aim of this study is showing how CFD can be used to simulate particle deposition/erosion in all the components of a turbine (i.e. fan, turbine, compressor), and predict the most critical regions of a given component. This will be done introducing the numerical models used for some applications, describing reference test cases, and showing/discussing results.